This disclosure relates generally to an apparatus and method for analyzing a breathing gas flowing along a breathing tubing for subject breathing. Liquid particles are delivered intermittently depending on a phase of the breathing cycle or continuously into the breathing gas. Also this disclosure relates to a corresponding arrangement.
A tidal volume (TV) is an amount of an air inspired or taken into lungs in a single breath. TV is dependent on the sex, size, height, age and health, et cetera, of a patient, but in general TV also decreases as the size of the patient decreases. In an average healthy adult, TV is about 400-600 ml whereas in an average healthy neonate, that measures 3.5-4 kg and is 50 cm tall, TV is approximately 25-50 ml. On the other hand, in an average premature neonate that measures only 500 grams TV is only about 2-3.5 ml. TV of a smaller patient is very difficult to measure, but it can be approximated to 4-7 ml/kg, applying a general rule of thumb for approximating the TV of the human lung. In practice, the TV of a patient suffering pulmonary system deficiency is normally much less than the approximation gives.
A respiration rate (RR) is dependent on the sex, size, height, age and health, et cetera, of the patient, but in general RR increases as the size of the patient decreases. In an average healthy adult, RR is about 10-20 breaths/minute, whereas RR of a neonate may exceed as high as 150 breaths/minute.
When the patient is mechanically ventilated with a conventional ventilator, an endotracheal tube is placed into a trachea so that it goes through the oral or nasal cavity and larynx. The other end of the endotracheal tube is connected to a breathing circuit Y-piece through a luer type connector. If the patient is gas monitored with a mainstream or sidestream gas analyzer, an airway adapter, used for sampling the breathing gas that is analyzed by the gas analyzer, is normally connected between the endotracheal tube and the breathing circuit Y-piece connectors. During an inspiration, the fresh breathing gas, including higher oxygen (O2) concentration, flows into the patient's lungs through an inspiratory limb of the breathing circuit Y-piece, the airway adapter, the endotracheal tube and their connectors, then to a trachea, a bronchus, a bronchi, bronchioles and finally reaching an alveoli deep in the lungs, where all the gas exchange actually occurs. Carbon dioxide (CO2) molecules in a hemoglobin of a blood flowing in tiny blood vessels around the alveoli are replaced with O2 molecules in the fresh breathing gas through the thin walls of the alveoli. O2 molecules take their place in the hemoglobin, whereas CO2 molecules flow out from the patient within the expired breathing gas, through the same path as the fresh gas came in during the inspiration. Thus a gas concentration of the breathing gas measured by the gas analyzer is somewhat proportional to the gas concentration in the blood.
A volume in a space between an intersection of the inspiratory and expiratory limbs of the Y-piece and the patient's mouth or nose, which is the beginning of oral and nasal cavities, is called a mechanical dead volume or dead space. A volume in a space between the patient's mouth or nose and the entrance of the alveoli is called an anatomical dead volume. The part of the lung that is injured or damaged for some reason and does not participate in the gas exchange is called a physical dead volume. As the used breathing gas flows out from the patient's lungs through the expiratory limb during expiration, a part of the used gas exits a pulmonary system, as well as the patient side of the breathing circuit, but remains in the mechanical and anatomical dead volume. Then as the fresh gas is inspired into the lungs through the inspiratory limb, the used gas already in the anatomical and mechanical dead volume flows into the lungs before the fresh gas. The used gas fills up some or all of the alveoli depending on a ratio of the dead volume and TV, or at least mixes with the fresh gas. This decreases the concentration of O2 as well as increases the concentration of CO2 in the lungs, which in turn decreases the gas exchange in the alveoli. This means that the larger the dead space, the larger the volume of the used gas, which has low O2 and high CO2 concentrations. As the used gas flows back to the patient's lungs during the inspiration, the gas exchange in the alveoli decreases. In other words, if the total dead volume is larger than TV, or as large as TV, the patient would not get any fresh gas into the lungs. Instead, the patient respires the used gas back and forth in the dead volume. In practice, a diffusion of gases assists the gas exchange over small dead volumes, especially when there is some movement of gases, such as high frequency ventilation evolves. However, the overall gas exchange in the alveoli would be lethal or dangerously poor.
The anatomical dead volume is very difficult to reduce, but it is proportional to the size and the physical condition of the patient. The mechanical dead volume depends on a breathing circuit design, an inner diameter of breathing circuit tubing, connectors and additional accessories, such as airway adapters used with a sidestream and mainstream gas analyzers. It is optimal that the mechanical dead space is zero as with normal breathing. The mechanical dead volume is more critical for smaller patients with smaller TV or patients suffering conditions such as, barotraumas, which decrease TV.
The mainstream gas analyzing is suitable for intubated patients or patients wearing a face or nasal mask. Mainstream analyzers are placed between the breathing circuit Y-piece and endotracheal tube, which is through their airway accessory used for measuring the gas concentration of the gas flowing through the analyzer. However, existing mainstream gas analyzers are big and heavy, and thus, very impractical to use. This is especially an issue with small patients, as the analyzer covers the patient's face and the analyzer's tiny endotracheal tube easily bends and clocks under the weight. Furthermore, accessories and additional connectors considerably add to the dead space, which is critical for a small patient with small TV. Also, the design of airway adapters and their non-tubular gas sampling chambers and connectors are inefficient. They generate turbulences in the breathing gas by mixing end tidal gas with fresh gas columns, thus mixing the gas samples that the mainstream analyzer tries to analyze, causing measurement inaccuracy, especially with higher RR and small TV. Currently, existing mainstream analyzers are not used with smaller patients.
The sidestream gas analyzers can be used with intubated and non-intubated patients. Sidestream analyzers are usually big and heavy and comprise complicated gas analyzing technology. As a result, they are placed further away from a patient and placed inside a host device, such as a patient monitor or ventilator. Gas samples are actively drawn into the analyzer with a gas pump through a sampling tube. When measuring intubated patients, the sampling tube is connected to a port in an airway adapter, which is placed between the breathing circuit Y-piece and an endotracheal tube. Gas samples are then drawn through the port, which is fluidly connected to the breathing gas flowing through the airway adapter. When measuring non-intubated patients, the sampling tube can be connected to nasal prongs, masks or straight into the nasal cavity to take gas samples from the gas flowing through the patient's upper airways.
The distance between the patient and the analyzer is usually very long, normally between 2-6 meters, which means gas samples travel a long distance before entering the analyzer. Gas samples drawn from the patient's breathing gas flow through the port in the airway adapter, through a connector between the sampling tube and the port, then through a 2-6 m long tiny tubing (inner diameter is usually between 0.8-1.5 mm), then through filters that separate water, mucus, and blood et cetera, and finally through membrane tubing. The membrane tubing has ionic properties that transfer water molecules through the membrane to even the humidity between the sample gas and the ambience. It is also possible to place filters between the port in the airway adapter and the sampling tubing followed by the membrane tubing to prevent liquids from entering the sampling tubing.
As gas samples travel through connectors, sample tubing and filters, the gas samples mix and average along the long path and therefore, comprise different concentrations. This considerably degrades the gas concentration measurement accuracy. Furthermore, the measurement accuracy degrades rapidly, especially when RR increases and TV decreases. This can be seen as damped and rounded capnogram, which is due to an increased number of smaller samples, or shortened gas columns that mix and average. This causes the amplitude of measured gas to decrease rapidly. The flow rate of sample gas has an effect on gas sample averaging. The lower the sample gas flow, the longer the gas columns travel through the tubing, et cetera, and the more they mix and average. Sample gas flow rates of existing sidestream gas analyzers are usually between 50-400 ml/min. As the flow rate of the sample gas is decreased, for example from 200 ml/min to 50 ml/min, the sensitivity to breathing gas concentration changes and decreases as the sample gas travels longer inside the tubing, et cetera, and mixes and averages more. It is possible to increase the flow rate of sample gas through the tube by decreasing the inner diameter of the tube. However, the negative pressure enabling the sample gas flow must be increased to keep the flow speed equal, which requires a more powerful pump. Gas columns may average even more as the flow speed of gas at the inner surface of the tube is zero and maximum in the middle of the tube. Smaller diameter tubing also tends to clog easier. For these reasons, most of the sidestream gas analyzer manufacturers specify the measurement range for RR only, which may go up to frequencies of 120-150 breaths/minute. However, the accuracy of the gas concentration measurement is not specified, or if it is specified, it only goes up to 15-60 breaths/minute, which is usable only for adults.
A nebulizer is a device used to deliver liquid form drugs into the patient's lungs in the form of a mist of small droplets called aerosol. Existing nebulizers are usually very big, clumsy, position sensitive and continuously produce the aerosol. Nebulizers are commonly placed between the inspiratory line, between the inspiratory limb of the breathing circuit Y-piece and the ventilator. Sometimes nebulizers are connected between the endotracheal tube and a manual resuscitator, especially when smaller patients are treated. However, this requires the patient to be naturally disconnected from the mechanical ventilator and ventilated manually. The aim is to produce the aerosol into the flowing inspiratory air to enable the aerosolized drug to enter the patient's lungs. However, as the existing nebulizers produce the aerosol continuously, during both inspiration and expiration and as the inspiration to expiration ratio of ventilation may be 1:1 or preferably 1:2, only a small part of the drug flows towards the patient's lungs.
When the nebulizer is connected between the inspiratory line, almost the whole line is continuously filled with aerosol continuously. During inspiration, only the gas column in the inspiratory line close to the Y-piece, which volume is proportional to patient's TV, flows towards the patient filling up the mechanical dead volume. When expiration starts, the aerosol in the dead volume flows towards the ventilator, but as the nebulizer produces the aerosol continuously, the aerosol produced during expiration flows straight out from the inspiratory line through the Y-piece and into the expiratory line. If existing nebulizers are placed between the breathing circuit Y-piece and the endotracheal tube, part of the aerosol produced during inspiration flows towards the patient, but aerosol generated during the expiration flows within the expired air away from the patient and into the ventilator and other devices connected to breathing circuit. Another problem with continuous aerosol production is similar to over humidification, where droplets produced into the motionless air come into contact with each other and combine into larger droplets. The droplets also come into contact with the breathing circuit walls, where they turn into liquid. Liquid then floats back and forth in the breathing circuit with the flowing air and may enter the patient's airways, which causes temporary airway obstruction, or even drowning. Liquid may also enter sensitive devices and analyzers connected to the breathing circuit, which causes malfunction, or even breakdown. There have been attempts for a nebulizer that can be turned on and off in regard to ventilation to increase the delivery efficiency, but such functioning devices do not currently exist on the market.
The functioning of other devices, such as sidestream gas analyzers that are connected between the breathing circuit Y-piece and the endotracheal tube, suffer from the aerosol or liquefied aerosol flowing back and forth within the flowing air. The optimum mean droplet diameter to ensure aerosol delivery into the deep lung and alveoli is between 1 and 5 microns. Thus, this size droplet easily enters the smallest cavities of devices connected to the breathing circuit. Aerosol flowing through the airway adapter disturbs the sidestream gas analyzers, especially during inspiration because they continuously draw gas samples and aerosol within the gas sample from the breathing circuit to generate a continuous real time capnogram of inspiratory and expiratory gases. Aerosol particles and liquefied aerosol easily flow into the sampling tubing, clogging the sampling tube and may even enter the filters and membrane tube, which may clog them as well. Many aerosolized drugs may include alcohol or other chemicals that may disturb and even destroy the functioning of filters and membranes when the aerosol or liquid may enter the very sensitive analyzing components, thus destroying the whole analyzer.
As a result, the existing sidestream gas analyzers are not very functional with existing nebulizers, as described earlier. Gas analyzing needs to be stopped and/or removed from the breathing circuit for the time of nebulization to prevent device malfunction. Gas analyzing also generates false measurement values during the nebulization that would further lead to incorrect estimation of a patient's condition and care practice.